The invention relates to power conversion apparatus and methods, and more particularly, to power conversion apparatus and methods for use, for example, in redundant power supply systems such as uninterrupted or uninterruptible power supplies (UPSs).
Power supply circuits are commonly used in equipment such as UPSs, motor drives, and other applications. Conventional UPSs use a variety of different circuit topologies, including standby, line-interactive and on-line topologies. Generally, each of these topologies has advantages and disadvantages and, accordingly, selection of a particular topology is typically governed by the needs of the application.
Many UPSs use an online topology. For example, a typical online UPS includes an AC/AC converter that produces an AC output voltage at a load from an AC input voltage provided by an AC power source such as a utility, using a DC link that can isolate the load from disturbance and other degradation of the AC power source. The intermediate DC bus is typically coupled to an inverter that inverts the DC voltage on the DC bus to produce an AC output. Other circuits, such as filters and regulators, may be included in the path with the rectifier and the inverter. Typically, the DC bus is also coupled to an auxiliary source of power, such as a battery, fuel cell and/or generator, which maintains the DC voltage on the DC bus in the event the AC power source fails. Some online UPSs use other circuit topologies, such as delta converters.
Under normal operating conditions, an online UPS supplies power to a load through a rectifier/inverter chain or other regulating circuitry, providing relatively clean and regulated power at the output of the UPS. When the AC power source fails, the UPS may achieve an uninterrupted transition to auxiliary power, as there typically is no need to change the state of a transfer switch. Some online UPSs also include a bypass feature such that, in the event of a failure and/or selection of this mode, the inverter is prevented from supplying power to the load (e.g., by disconnecting it from the load or placing it into an inactive standby state) while the load is directly coupled to the AC power source via a bypass path. Such a feature may be used to provide an “economy” mode of operation, as power dissipation associated with the operation of the rectifier/inverter chain may be reduced when the load is transferred to the bypass path.
However, such an approach may be compromised if the AC power source is subject to small but frequent excursions outside of the power quality criteria, as may be the case, for example, in systems powered by an unsteady AC power source, such as an auxiliary generator set. In such a case, the UPS may be forced to power the load through the inverter in order to meet the power quality requirements of the load, thus reducing the opportunity for greater efficiency through bypass operation. Techniques for controlling power factor and other power quality parameters in a higher efficiency mode of a UPS are described in U.S. Pat. No. 6,295,215 to Faria et al.